The present invention is directed generally to the field of seals for use in statically and dynamically sealing an annular cavity formed between two concentric tubes and more particularly to a restricter incorporated into a multi-element seal assembly to assist in restricting temperature-induced radial growth of the seals.
Seals take many shapes and forms and play a critical role in many devices. They are employed in settings wherein the components between which the seals are used are either static or dynamic with respect to one another. Moreover, the environment in which such seals are used may present extreme conditions such as high or low temperatures or transitions between the two, high pressure, friction, and chemical exposure. Seals used in this environment have a very short life, often failing after only a small number of cycles. Materials are therefore strategically selected to address one or more of the environmental conditions. One material may not address all conditions, but the combination of several seals made from different materials will typically address the majority of environmental conditions encountered.
A significant problem in designing seal assemblies for environments where conditions vary widely is that the materials selected for the seals may not all react in the same way to the environmental conditions. Such reactions may, in fact, be adverse to their sealing function. This difficulty is acutely manifested where extreme temperature changes are encountered. In such cases, the compensation for thermal conditions must be so great that if the seal is designed primarily to seal at higher temperatures, it loses its ability to seal at lower temperatures and vice versa.
Exemplary of these settings where such vexing environmental conditions are encountered are tools for use in subterranean downhole wells. Following the drilling of a downhole well, a string of casing is cemented in place to form an outer housing for the well hole. The casing is then perforated to permit the flow of fluids into the interior of the casing. Fluids are extracted from the casing via a string of conduits called production tubing or work tubes which are suspended concentrically within the casing. To permit the efficient extraction of fluids from the casing via the work tubes, or to permit the infusion of chemical inhibitors, stimulants or the like into the well hole, the work tubes are provided with a downhole well tool, generally located deep within the well, which acts as a valve to control the communication of fluids between the interior of the work tubes and the annular region between the work tubes and the casing.
Downhole well tools are well known in the drilling/extraction industry. Such downhole well tools are disclosed in, for example, U.S. Pat. No. 5,263,683 issued to Wong, entitled "Sliding Sleeve Valve," U.S. Pat. No. 5,316,084 issued to Murray et al. and U.S. Pat. No. 5,156,220 issued to Forehand et al., both entitled "Well Tool With Sealing Means." Such downhole well tools generally are provided with an outer housing which is an outer, generally tubular member, having threads on each end for connection to the work tubes and have a port or series of ports in the outer housing, generally arranged in a circumferential pattern around the midsection of the housing. Positioned concentrically and slidably within the housing is an inner, generally tubular member or sliding member, also having a port or series of ports arranged in a circumferential pattern around its midsection. The annular region between the outer housing and the sliding member is sealed at its upper end, above the housing ports, by a seal and at its lower end, below the housing ports, by another seal.
The valving function of the downhole well tool is accomplished by moving the sliding member longitudinally within the outer housing such that the ports of the sliding member are moved into and out of fluid communication with the housing ports. The sliding member is manipulated between the open and closed positions by means of a wireline, remedial coiled tubing, electric line, or any other well known mechanism controllable from atop the well hole. To permit fluid communication between the region within the work tubes and within the annular region outside the work tubes and within the casing, the sliding member is thus slidably moved to a position whereby the ports of the sliding member are located between the seals located above and below the housing ports. To discontinue or prevent fluid communication between the interior of the work tubes and the exterior of the work tubes, the sliding member is positioned whereby the ports of the sliding member are not located between the seals above and below the housing ports.
Essential to the valving function of the downhole well tool is a reliable sealing engagement between the sliding member and housing both above and below the ports on the housing.
Prior attempts to provide seals capable of withstanding the high temperatures and broad temperature ranges present in the down hole well environment have included the use of various types of polymeric material. Although polymeric materials have proven to be chemically resistant, after prolonged exposure to the high temperatures and broad temperature ranges present within the well hole, seals made from such materials will harden, become brittle, and will fail to provide sealing engagement between the sliding member and housing.
A significant improvement over single-seal designs is provided by prior art designs employing a combination of individual seal elements in a single seal assembly. Examples of such "nested" or multi-element seal assemblies generally known in the art are disclosed in U.S. Pat. No. 4,576,385 issued to Ungchusri et al., entitled "Fluid Packing Assembly With Alternating Diverse Seal Ring Elements" and in U.S. Pat. No. 5,309,993 issued to Coon et al., entitled "Chevron Seal for a Well Tool," the latter of which is incorporated herein in its entirety by reference. The advantage of such nested seal assemblies is that they permit the designer to combine seals made from several different materials into a single sealing unit. The materials employed can include a combination of metallic and non-metallic materials.
Nested seal assemblies provide the designer with the ability to partially compensate for the widely ranging conditions present in sealing applications, particularly in downhole wells. Whereas some of the individual seal components will function better at lower temperatures, others will function better at higher temperatures.
Thus, the purpose of using seal assemblies is to increase sealing efficiency relative to individual seal elements and to provide the opportunity to combine different types of seals and materials to accomplish sealing under a wide range of environmental conditions.
One significant drawback to any of the prior art seal assemblies, however, including nested seal assemblies, is that high temperatures and broad temperature ranges within the well bore, and of the downhole well tool itself, cause a large degree of thermally-induced growth in the individual seals and in the seal assembly as a whole. This thermally-induced seal growth occurs along the longitudinal axis of the downhole well and tool and in the radial (i.e., perpendicular to the longitudinal axis of the downhole well) direction. While longitudinal growth is not a particularly relevant factor insofar as seal longevity is concerned due to the ability to effectively restrain such growth, radial growth presents great challenges. Radial growth of seals results from the use of seal materials having high coefficients of thermal expansion.
When designing for sealing in environments where large variations in temperature occur, the degree of thermally-induced radial growth can be compensated for during design by sizing the various elements according to the amount of radial growth anticipated. However, as temperatures increase, or as the temperature range increases, compensation using sizing alone is insufficient to accommodate the degree of thermal growth that accompanies such conditions. This is due to the fact that seals cannot be sized to seal only at higher temperatures because the seals would not be capable of sealing at lower temperatures. Alternatively, sizing seals to accommodate the sealing function at lower temperatures can create a situation whereby thermally-induced radial growth creates too much interference at high temperatures. Such interference can cause seal damage or, in more extreme cases, can cause the downhole well tool to seize or lock up, whereby the sliding member cannot slide within the housing, thus leading to costly down time. Moreover, because seal assemblies may include seals made from disparate materials, thermally-induced radial growth is inconsistent among the various seal elements, thus complicating the design process.
The present invention improves on the seal assembly concept by providing a mechanism which restricts the thermal growth of adjacent seals in a seal assembly. The result is that, regardless of the non-metallic material used to make the seals, thermal growth of each seal is restricted, thus leading to greatly improved sealing capacity of the seal assembly, greater reliability, and lower operating costs.